This invention relates to phase shift mask blanks and phase shift masks for use in the fabrication of semiconductor integrated circuits, and more particularly, to phase shift mask blanks and phase shift masks of the halftone type wherein light of exposure wavelength is attenuated by the phase shift film. It also relates to a method of manufacturing such phase shift mask blanks and phase shift masks.
Photomasks are used in a wide variety of applications including the fabrication of semiconductor integrated circuits such as ICs, LSIs and VLSIs. Basically, the photomask is prepared from a photomask blank having a chromium based light-shielding film on a transparent substrate, by forming a predetermined pattern in the light-shielding film by photolithography using UV or electron beams. The current demand for a higher level of integration in the semiconductor integrated circuit market has created a need for a smaller pattern rule. The traditional solution is by reducing the wavelength of exposure light.
However, reducing the wavelength of exposure light improves resolution at the sacrifice of focal depth. This lowers the process stability and adversely affects the manufacture yield of products.
One effective pattern transfer method for solving the problem is a phase shift method. A phase shift mask is used as a mask for transferring a micro-pattern.
Referring to FIGS. 9A and 9B, a phase shift mask, specifically a halftone phase shift mask is illustrated as comprising a substrate 32 and a phase shifter 34 forming a pattern on the substrate, the substrate 32 being uncovered where the phase shifter 34 is absent. A phase difference of about 180xc2x0 is set between light transmitted by the uncovered area of substrate 32 and light transmitted by the phase shifter. Due to light interference at the pattern boundary, the light intensity at the interfering boundary becomes zero, improving the contrast of a transferred image. The phase shift method permits to increase the focal depth for acquiring the desired resolution. This achieves improvements in resolution and exposure process margin, as compared with conventional masks having ordinary light-shielding patterns in the form of chromium film.
Depending on the light transmission of phase shifter, the phase shift masks are generally divided for practical application into full transmission type phase shift masks and halftone type phase shift masks. The full transmission type phase shift masks are transparent to the exposure light wavelength because the light transmittance of the phase shifter section is equal to the light transmittance of uncovered substrate areas. In the halftone type phase shift masks, the light transmittance of the phase shifter section is several percents to several tens of percents of the light transmittance of uncovered substrate areas.
FIGS. 1 and 2 illustrate the basic structure of a halftone type phase shift mask blank and a halftone type phase shift mask, respectively. The halftone type phase shift mask blank shown in FIG. 1 has a halftone phase shift film formed over substantially the entire surface of a transparent substrate 1. Patterning the phase shift film 2 results in the halftone type phase shift mask which is shown in FIG. 2 as comprising phase shifter sections 2a forming the pattern on the substrate 1 and uncovered areas 1a of the substrate where the phase shifter is absent. Light that passes the phase shifter section 2a is phase shifted relative to light that passes the uncovered substrate area 1a. The transmittance of the phase shifter section 2a is set to a light intensity that is insensitive to the resist on the transferred substrate. Accordingly, the phase shifter section 2a has a light-shielding function of substantially shielding exposure light.
The halftone type phase shift masks include single-layer halftone type phase shift masks featuring a simple structure and ease of manufacture. Some single-layer halftone type phase shift masks known in the art have a phase shifter of MoSi base materials such as MoSiO and MoSiON as described in JP-A 7-140635.
What is important for such phase shift masks and phase shift mask blanks is the control of optical properties at the exposure wavelength including transmittance, reflectance and refractive index. In particular, optical properties are largely affected by the film composition.
The phase shift masks and phase shift mask blanks having the molybdenum silicide (MoSi) based phase shifter are generally manufactured by a reactive sputtering technique. The reactive gas used for manufacture often contains oxygen gas or nitrogen monoxide gas as an oxygen source. When MoSiO or MoSiON is deposited as the MoSi based phase shift film using such reactive gas, there arises a problem that optical properties including transmittance, reflectance and refractive index often become non-uniform within the plane of the substrate. This phenomenon occurs because the oxidizing gas such as oxygen gas or nitrogen monoxide gas is so reactive that more reaction takes place near the gas inlet port and the quantity of available gas decreases as it flows away from the gas inlet port. Also because of the high reactivity, optical properties rather become sensitive to variations in gas flow rate, which is undesirable for consistent mass manufacture.
An object of the invention is to provide a phase shift mask blank and a phase shift mask of quality which has fully uniform optical properties within the plane of the substrate. Another object of the invention is to provide a method for manufacturing a phase shift mask blank and a phase shift mask, which is easy to control during deposition of a phase shift film and enables consistent manufacture.
The invention pertains to a phase shift mask blank comprising a substrate transparent to the wavelength of exposure light and at least one layer of phase shift film on the substrate. It has been found that when the phase shift film is formed of molybdenum silicide oxycarbide or molybdenum silicide oxynitride carbide by a sputtering technique using a target of molybdenum silicide and a sputtering gas containing carbon dioxide as an oxygen source and optionally, nitrogen gas as a nitrogen source, the phase shift film has fully uniform optical properties within the substrate plane. The phase shift mask blank is of quality as well as a phase shift mask obtained from the blank. The method is easy to control during deposition of the phase shift film and enables consistent mass manufacture.
In a first aspect, the invention provides a phase shift mask blank comprising a transparent substrate and at least one layer of a phase shift film on the substrate. The phase shift film is formed of molybdenum silicide oxycarbide or molybdenum silicide oxynitride carbide.
In a second aspect, the invention provides a phase shift mask blank comprising a transparent substrate, at least one layer of phase shift film on the substrate, the phase shift film being formed of molybdenum silicide oxycarbide or molybdenum silicide oxynitride carbide, and a chromium-based light-shielding film, a chromium-based antireflection film or a multilayer combination of both on the phase shift film. The chromium-based light-shielding film or antireflection film is preferably formed of chromium oxycarbide or chromium oxynitride carbide.
In either embodiment, the phase shift film shifts the phase of exposure light passing through it by 180xc2x15 degrees and has a transmittance of 3 to 40%.
In a third aspect, the invention provides a phase shift mask manufactured by patterning the phase shift film on the phase shift mask blank.
In a fourth aspect, the invention provides a method of manufacturing a phase shift mask blank comprising a transparent substrate and at least one layer of phase shift film on the substrate, the method comprising the step of forming the phase shift film by a sputtering technique using a target of molybdenum silicide and a sputtering gas containing carbon dioxide as an oxygen source and optionally, nitrogen gas as a nitrogen source. Preferably, the phase shift film shifts the phase of exposure light passing through it by 180xc2x15 degrees and has a transmittance of 3 to 40%.
The method may further include the step of forming a chromium-based light-shielding film, a chromium-based antireflection film or a multilayer combination of both on the phase shift film by a sputtering technique using a target of chromium alone or in admixture with at least one of oxygen, nitrogen and carbon.
In a further aspect, the invention provides a method of manufacturing a phase shift mask, comprising the steps of lithographically forming a patterned resist film on the phase shift film of the phase shift mask blank obtained by the method of the fourth aspect; removing areas of the phase shift film which are not covered with the resist film, by an etching technique; and removing the resist film.
In a still further aspect, the invention provides a method of manufacturing a phase shift mask, comprising the steps of etching away areas of the chromium-based light-shielding or antireflection film or multilayer combination obtained by the above method where exposure to light is necessary, so as to leave corresponding areas of the phase shift film exposed on the surface; lithographically forming a patterned resist film on the phase shift film; removing areas of the phase shift film which are not covered with the resist film, by an etching technique; and removing the resist film.
When an oxidized film of molybdenum silicide is deposited on a substrate by a reactive sputtering technique, the invention uses carbon dioxide gas, which is less reactive than oxygen and nitrogen monoxide gas, as the oxidizing gas. Because of the low reactivity, the carbon dioxide gas can spread around uniformly over a wider extent so that the MoSi base phase shift film deposited has uniform optical properties within the substrate plane.
Also because of the low reactivity, the carbon dioxide gas allows for a greater margin against unexpected variations of many parameters associated with the deposition process. Then the MoSi base phase shift film can be deposited in a stable and controllable manner.
In the embodiment wherein a phase shift film is formed of molybdenum silicide oxycarbide or molybdenum silicide oxynitride carbide and a chromium-based light-shielding film, a chromium-based antireflection film or a multilayer combination of both is formed on the phase shift film, they cooperate to allow for precise patterning to a finer rule. The phase shift mask lends itself to the fabrication of semiconductor integrated circuits to a smaller minimum feature size and a higher level of integration.